The Petroleum Safety Authority Norway (PSA) said it had found failings in BP’s maintenance and management systems, and ordered it to review its procedures.

Initial investigations had already discovered inadequate fire and explosion protection on the platform, which BP was forced to rectify last year.

It concluded "that the incident had the potential to become a major accident, with the risk that a number of lives might have been lost and substantial material damage caused".

BP had already been ordered to overhaul its system of maintenance for its ageing North Sea infrastructure after a fire on its Valhall platform in July 2011, which the PSA concluded also could have been fatal. Yet the PSA said on Monday that its investigation into the Ula leak found "deficiencies still exist in the maintenance system".

Norwegian police confirmed they were still investigating the Ula incident to see whether BP had committed any crimes, which could potentially lead to penalties.

The damning findings from the PSA come just over three years since the explosion on BP's Deepwater Horizon rig in the Gulf of Mexico, which killed 11 men and caused the worst offshore oil spill in US history. BP has insisted that it has overhauled its safety policies since the disaster.

In a statement on Monday, the PSA said the Ula leak has been caused by corrosion that caused bolts holding together a valve to break.

The leak took place in a "separator outlet", which divides crude oil, gas condensate and gas. "Seepage in the valve exposed the bolts to produced water with a high content of chlorides and a temperature of about 120°C. This resulted in chloride stress corrosion cracking which weakened the bolts until they finally fractured," the PSA said.

"The investigation has identified a number of serious breaches of the regulations, related in part to BP’s management system for activities on the Norwegian continental shelf."

Oil and gas are both highly flammable, meaning even relatively small leaks can cause catastrophic explosions.

BP said 140 people were on the linked series of Ula platforms at the time of the incident, five of whom were on the production platform where the leak occurred.

Although no-one was hurt, production was shut down for 67 days after the incident.

After the investigation into the 2011 Valhall fire, BP was ordered to conduct a "review and assessment of the company’s systems for maintenance management on aging installations, and ensuring that maintenance programmes and the execution of such work were tailored to the age and condition of the installations and equipment".

Having found failings again in the Ula investigation, the PSA ordered BP to "review its management system" for Norway.

BP must assess whether the system was "adequate for identifying and managing risk" and "why the system has not been adequate for identifying and dealing with the nonconformities identified in the investigation of the leak on Ula".

It was ordered to "assess whether measures planned and initiated after the fire on Valhall in 2011 and other improvement activities are relevant and collectively adequate in light of the nonconformities identified following the leak on Ula".

A BP spokesman said: “BP has received the PSA’s investigation report following the incident on Ula last year. The findings closely match BP’s own investigation and work has already started to address the issues raised and to fully comply with the orders by the end of 2013.”